Feature parity with AlgebraOfGraphics

[piever]

This is an issue to track what is needed before essentially replacing the code in AlgebraOfGraphics with the code developed here. As the surface syntax ended up being somewhat similar, this new approach can essentially just be AlgebraOfGraphics 0.4

TODO

• [x] broadcast semantics for wide-formatted tables (https://github.com/piever/SplitApplyPlot.jl/pull/25)
• [ ] broadcast semantics for general high-dimensional arrays (no data frame involved), eg, data is in a Array{Float64, 3} (probably not a blocker)
• [x] Support using integers to select columns (part of https://github.com/piever/SplitApplyPlot.jl/pull/27)
• [x] legend implementation (#32)
• [x] missing analyses (#37)
• [x] something like draw that automatically add opinionated defaults (could probably be just plot + facet!, as in https://piever.github.io/SplitApplyPlot.jl/dev/generated/gallery/#Faceting)
• [x] dodge and stack (for bars, should work out of the box with new AbstractPlotting, but it is also needed for boxplot and violin)

[piever]

For complete feature parity, it would also be good to double check that labeling is as flexible. In particular we may want to avoid collapsing names in the Entries object, but only in the AxisEntries object, as different axes are allowed to have different labels in AoG.